1. Field of the Invention
The present invention relates to a syringe for collecting a liquid sample. While the syringe according to the invention may be used for collecting samples from liquids of any type, it is especially suited for collecting a blood sample, such as an arterial blood sample, from a blood vessel for subsequent blood gas analysis.
The blood parameters determined by blood gas analysis, such as the partial pressure of oxygen (pO.sub.2), the partial pressure of carbon dioxide (pCO.sub.2), and the acidity (pH), may be influenced and changed when the blood sample comes into contact with the ambient atmosphere, and, consequently, it is necessary to take special measures in order to avoid or reduce such contact.
2. Description of the Prior Art
Such blood collection syringes, wherein the blood collecting chamber is vented to the atmosphere through a venting passage while a blood sample is collected, and wherein the venting passage may be closed when a suitable amount of blood has been collected in the syringe, are well known, for example from U.S. Pat. Nos. 3,943,917, 4,133,304, 4,206,768 and 4,257,426 and German Offenlegungsschrift No. 3,041,563. However, all of these known blood collecting syringes require certain manual operations subsequent to the collection of a blood sample in order to close the venting passage.
U.S. Pat. Nos. 3,960,139, 3,978,846, 4,266,558, 4,266,559, 4,327,745, 4,340,067, and 4,373,535, PCT publication No. WO 81/03426, and published European patent applications Nos. 47,176 and 47,806 all disclose blood samplers comprising a collecting chamber, which is vented to the atmosphere through a filter or closure element, which is pervious to gas, but impervious to liquid or becomes impervious to liquid when wetted thereby. This latter structure causes the flow of blood sample into the blood collecting chamber of the syringe to be automatically stopped, when the chamber has been filled so that blood comes into contact with the filter or closure element.
In most of these prior art syringes the venting passage opens into the blood collecting chamber of the syringe at such a position or such positions that a complete discharge of air or gas from the blood collecting chamber is dependent on the position in which the syringe is held when a blood sample is collected. The prior art also comprises a syringe disclosed in U.S. Pat. No. 4,299,238.
However, the above mentioned U.S. Pat. No. 4,340,067 discloses a syringe comprising a piston, which is provided with a peripheral sleeve of a hydrophilic material, such as porous filter paper. This porous sleeve forms part of a venting passage and allows air or gas to escape from the sample collecting chamber in any rotational position of the syringe provided that the piston rod is upwardly directed. When blood comes into contact with the hydrophilic sleeve surrounding the piston, the hydrophilic material swells and closes the venting passage defined between the cylinder wall and the outer peripheral wall of the piston. In order to reduce the risk that blood passes the piston, so that the user of a syringe may come into contact with such blood, the hydrophilic piston sleeve used in the known syringe could advantageously have been replaced by a sleeve made from a hydropholic material.
However, despite the kind of porous material from which the piston sleeve is made, this prior art syringe has the disadvantage that it cannot be used for creating subatmospheric pressure within the blood collecting chamber, for example when the blood pressure in an artery from which a blood sample is to be collected, is insufficient for causing blood to flow through the hollow needle of the syringe into the blood collecting chamber.